Gain equalizers are used in applications that require signal level control. Level control can be accomplished by either reflecting a portion of the input signal back to its source or by absorbing some of the signal in the equalizer itself. The latter case is often preferred because the mismatch which results from using a reflective equalizer can create problems for other devices in the system such as nonsymmetrical two-port amplifiers. It is for this reason that absorptive passive components are more popular, particularly in microwave applications.
Variations in temperature can affect various component parts of a microwave system causing differences in signal strengths at different temperatures and frequencies. Much time, effort and expense has gone into the design of components of such systems in an effort to stabilize them over various temperature and frequency ranges. This has greatly increased the cost of microwave systems that must be exposed to wide temperature ranges. A gain equalizer is a passive component which solves this issue by flattening the linear increase in attenuation or (decrease in gain) with frequency and temperature. In order to achieve this, the gain equalizer utilizes thermistors with resistances values that change over temperature.
One example of a gain equalizer is an absorptive-type temperature variable attenuator is the attenuator described in U.S. Pat. No. 5,332,981 entitled, “Temperature Variable Attenuator,” which is incorporated herein by reference. Examples of the attenuator of the '981 patent include a Tee attenuator and a Pi attenuator. In each case at least one resistor has a temperature coefficient of resistance (TCR) that is different from that of the others such that the attenuation of the attenuator changes a controlled rate with changes in temperature while the impedance of the attenuator remains within acceptable levels.